Drilling fluids are reputed as the blood for bore wells. Good drilling fluid techniques are one of the important guarantees for safe, high-quality, efficient, and quick well drilling production. Drilling fluids are required in the development of oil fields. As the technology makes progresses in the petroleum industry, the exploration and development of low-permeability oil and gas resources has taken the first place among the five investment priorities in the development in the upstream of oil and gas industry in the world. Low-permeability oil and gas resources are very abundant and distributed widely in the world. A large quantity of low-permeability oil and gas fields have been found in major oil producing countries such as USA, Russia, and Canada, etc. The proved geological reserves of low-permeability oil and gas resources account for more than ⅓ of the proved geological reserves of oil and gas resources in China, and are widely distributed in the major oil fields. In consideration of energy safety in China, it is expected that a substantial part of new reserve and yield will be obtained from low-porosity and low-permeability oil and gas reserves.
The permeability of low-permeability reservoirs is 10˜100 mD (milli Darcy); the permeability of ultra-low permeability reservoirs is lower than 10 mD. Compared with medium-permeability and high-permeability oil and gas reservoirs, it is much more difficult to explore and develop low-permeability oil and gas reservoirs. Low-porosity and low permeability reservoirs usually have characteristics such as thin pore throats, poor permeability, high clay mineral content, high capillary pressure, severe heterogeneity, and developed natural fractures, etc. Hence, reservoir protection is particularly important. To improve the effects and benefits of exploration and development of such reservoirs, it is particularly important to take effective reservoir protection measures.
Effective reservoir protection is an effective guarantee for improving the rate of oil and gas recovery, so that the skin factor can be an optimal negative value. If the drilling fluid is designed or used inappropriately in the well drilling process, the liquid and solid in the drilling fluid may intrude into the oil reservoir and have physical and chemical reactions with clay and other minerals in the oil and gas layer. Consequently, the permeability in the oil layer in the immediate vicinity of the well may be decreased severely, and the resistance against air and gas flow towards the bottom of the well may be increased, resulting in reduced oil yield. The injuries to the reservoir have critical impacts on low-permeability reservoirs.
The optimization of drilling fluid system and formulation is one of the important considerations in reservoir protection in the drilling process. Effective control of rheological property and filter loss property of the drilling fluid, high compatibility with the reservoir, design of temporary plugging scheme, and formation of dense filter mass that can be removed easily are key factors in the optimization and design of a reservoir protective drilling fluid. In addition, developing novel, easy-to-operate, and effective reservoir protective techniques is a new task in the reservoir protective drilling fluid field.
Low/ultra-low permeability reservoirs usually have characteristics such as high argillaceous cement content and high capillary pressure, thin pore throats, complex structure, severe heterogeneity, and high oil and gas flow resistance, etc., and may easily have damages such as water sensitivity and water blocking, etc. resulted from intrusion of foreign fluids in the drilling process. The damage ratio is as high as 70%˜90%. It is proven in field tests that the above problem can't be solved by conventional drilling fluid systems. Hence, it is especially important to develop new drilling fluid additives and reservoir protective drilling fluid techniques to meet the current challenge of complex geological conditions. Up to now, though long-time researches on reservoir protective drilling fluid techniques have been made in worldwide, and techniques such as shielded temporary plugging technique, fractal geometry-based temporary plugging technique, D90 ideal temporary plugging technique, broad-spectrum temporary plugging technique, alkali soluble micrometer-level cellulose temporary plugging technique, D50 temporary plugging technique, filming technique, oil film technique, etc., have been developed, the protective effect of these techniques is not ideal for complex reservoirs such as low/ultra-low permeability reservoirs. These techniques have to be improved further. The root cause is that these techniques don't take consideration of the reservoir damages such as water blocking and water sensitivity incurred by spurt loss, and the plugging effect of these techniques should be further improved under some conditions.
In addition, for water blocking damage in low-permeability gas reservoirs, though an ideal of adding surface active agents into drilling fluids to relieve water blocking damage has been put forward and applied in the Qiudong Low-Permeability Gas Reservoir, which provides a new way for improving the drilling fluid and completion fluid techniques for protection of low/ultra-low permeability reservoirs, further improvements must be made in selection of surface active agent or establishment of protection. The existing drilling fluids have properties that can essentially meet the requirement for cuttings carrying and well wall stability, but don't provide an ideal reservoir protection effect, and the core permeability recovery value of the reservoir is low. Though temporary plugging agents are added in these drilling fluids, the plugging performance is not high because the structural characteristics of low/ultra-low permeability reservoirs are not taken into consideration fully. Consequently, the filtrate may intrude into the reservoir and thereby results in damages such as water sensitivity and water blocking, etc.